The present invention relates generally to the field of holography. More particularly, it concerns methods and devices for creating and printing variable size and variable resolution holographic stereograms and holographic optical elements using computer rendered images of three-dimensional computer models or using computer processed images.
A holographic stereogram is a type of hologram synthesized or composed from a set of two-dimensional views of a subject. A holographic stereogram is capable of creating the convincing illusion of a solid three-dimensional subject from closely spaced, discrete-perspective, two-dimensional component views. In addition, if the two-dimensional component views are properly generated, a holographic stereogram can also create the illusion of an animated image. Although holographic stereograms can project such special effects, due to limitations in the methods and techniques for printing holographic stereograms, holographic stereograms have generally been expensive, difficult, and time consuming to produce.
Techniques have been developed for reducing the number of steps involved in producing holographic stereograms to one optical printing step. One-step technology usually involves using computer processed images of objects or computer models of objects to build a hologram from a number of contiguous, small, elemental pieces, known as elemental holograms or hogels. This one-step technology eliminates the need to create a preliminary hologram.
To produce a full-parallax, holographic stereogram using traditional one-step technology, a three-dimensional computer model of an object or a scene is created. There are numerous computer graphic modeling programs, rendering programs, animation programs, three dimensional digitalization systems, or combinations of the programs or systems that can be used to generate and manipulate a three-dimensional computer model of an object or a scene. Examples of such programs or systems include, but are not limited to, computer-aided-design (CAD) programs, scientific visualization programs, and virtual reality programs.
In addition, to produce a holographic stereogram using one-step technology requires that the position of the hologram surface and individual elemental holograms relative to an object or a scene be determined. Furthermore, a proper computer graphic camera(s)'s description for an elemental hologram and the size and location of a spatial light modulator (SLM), a device that can display a two-dimensional image, need to be determined.
Once all the aforementioned initial parameters are determined, a two-dimensional projection on the SLM for each elemental hologram is computed based on the computer graphic model of the object or scene that was created, the positions of the elemental holograms, and the computer graphic camera's description for the elemental holograms. The two-dimensional projection on the SLM for each elemental hologram may be rendered using various computer graphic techniques. The process of creating two-dimensional views from a three-dimensional object and adding qualities such as variations in color and shade to a computer graphic model is often referred to as rendering. There are numerous methods for rendering. One method is ray-tracing, which computes images by accurately simulating sampled light rays in a computer model. Another method is scan-line conversion, which computes images one raster or line at a time. Typically scan-line rendering does not produce as realistic results as ray tracing. However, scan-line rendering is frequently used in animation packages because it is faster. Another method for using computer graphics to render images for one-step, full-parallax holographic stereograms is described in an article by Halle and Kropp. Halle, M. and Kropp, A., “Fast Computer Graphics Rendering for Full Parallax Spatial Displays,” Proc. Soc. Photo-Opt. Instrzetn. Eng. (SPIE), 3011:105-112 (Feb. 10-11, 1997), the disclosure of which is incorporated herein by reference.
When holographic stereograms are produced by either the multi-step or one-step techniques, the reconstructed images may have geometric image distortions. These geometric image distortions may be very apparent, especially in large, billboard size holographic displays or holographic displays in other geometries, such as an alcove or a partial cylinder.
One solution that has been incorporated into multi-step techniques to correct for geometric image distortions for multiplex holograms is discussed in an article by Okada. Okada, K., et. al., “A Method of Distortion Compensation of Multiplex Holograms,” Optics Communications, vol. 48, no. 3, pp. 167-170 (Dec. 1, 1983), the disclosure of which is incorporated herein by reference. The technique discussed in Okada's article to correct distortion is a method to correct geometrical and time distortion of a single or monocular viewpoint of a finished hologram. Because it is a post-processing method that takes place after image acquisition, Okada's technique would be inefficient if adopted to generate animated computer graphics for one-step, holographic stereograms. Moreover, Okada's method only produces horizontal-parallax-only transmission type holograms.
Others have developed techniques for pre-distorting one-step, holographic stereograms to reduce distortion in the final holographic display. One such pre-distortion technique is described in a paper by Halle and others. Halle, M. et al, “The Ultragram: A Generalized Holographic Stereogram,” Proc. Soc. Photo-Opt. Instrum. Eng. (SPIE), vol. 1461, Practical Holography V, p. 142 (February 1991), the disclosure of which is incorporated herein by reference. Although widely used, typical pre-distortion techniques for one-step methods for producing full-parallax, holographic stereograms are significantly limited by available computer processing speeds and the resolution of images produced by traditional one-step methods. In addition, techniques for pre-distorting one-step, full-parallax, holographic stereograms have not been able to produce comprehensible, animated, one-step, full-parallax, holographic stereograms.
Apparatus for printing one-step, monochromatic, holographic-stereograms have been developed. Typically, such prior art printers, as depicted in FIG. 1, include: a monochrome coherent light source 1, lenses 42, mirrors 40, an optical system 89, a shutter 10, a mechanism for translating film 69, holographic recording material 70, usually in the form of film, a personal computer 85 to control the timing for the exposure sequence, and a separate high-speed computer 87 for image calculations. The prior art printer depicted in FIG. 1, was discussed in two articles by Yamaguchi. Yamaguchi, M. et al., “Development of a Prototype Full-Parallax Holoprinter.” Proc. Soc. Photo-Opt. Instrzun. Eng. (SPIE), vol. 2406, Practical Holography IX, pp. 50-56 (February 1995); and Yamaguchi, M., et al., “High-Quality Recording of a Full-Parallax Holographic Stereogram with a Digital Diffuser.” Optics Letters, vol. 19, no. 2, pp. 135-137 (Jan. 20, 1994), the disclosures of each are incorporated herein by reference. The prior art printer depicted in FIG. 1 is capable of producing monochromatic holographic stereograms, but not full-color holographic stereograms.
A typical prior art hologram printer, like the one depicted in FIG. 1, usually is supported by a vibration isolation table 80. In addition, the prior art printer depicted by FIG. 1 uses a HeNe laser for a light source 1 that produces a coherent light beam 5 that may be collimated. A shutter 10 is placed at the output of light source 1. A beam-splitter 15 splits the light 5 from the light source 1 into an object beam 20 and a reference beam 25. The polarization of the object and reference beams 20, 25 are adjusted by a pair of half-wave plates 30 and a pair of polarizers 35. The half-wave plates 30 and polarizers 35 also control the ratio of the beams. The prior art printer also uses a number of mirrors 40. In addition, the prior art printer uses a system of enlarging lenses 42 to distribute the object beam 20 from the light source 1 into the optical system 89 depicted in FIG. 1.
The optical system 89 of the prior art printer of FIG. 1 includes a band-limited diffuser 45, a liquid crystal display panel (LCD panel) 50, and a converging lens 55. A band-limited diffuser is a diffuser with a deterministic phase pattern designed to diffuse light in a specific pattern or direction. The band-limited diffuser 45 depicted in FIG. 1 is specifically designed for the monochromatic light source being used—a HeNe laser. The LCD panel 50 used in the prior art printer of FIG. 1 is a gray scale, electrically addressed panel with twisted-nematic liquid crystals. The LCD panel 50 receives image data calculated by a high-speed computer 87 by an analog video signal. The converging lens 55 shown in FIG. 1 focuses the images from the LCD panel 50 to the holographic recording material 70. The converging lens 55 generally has a low f-number in order to produce a wide angle of view. Due to the need to correct for spherical aberrations along the optical axis, Yamaguchi utilized a converging lens 55 composed of three lenses to reduce spherical aberration and realize a f-number of around 0.8.
To prevent the exposure of parts of the holographic recording material 70 that are not part of the elemental hologram 110 meant to be exposed, the prior art printer of FIG. 1, uses, in close proximity to the holographic recording material 70, an object beam masking plate 60 with an aperture the size of the elemental hologram 110 to prevent the object beam 20 from exposing other parts of the holographic recording material 70.
The band-limited diffuser 45 shown in FIG. 1 improves the uniformity of the distribution of the object beam 20 over an elemental hologram on the holographic recording material 70. If the band-limited diffuser 45 is designed such that an object beam 20 is focused only over the area of an elemental hologram, then an object beam masking plate 60 is not needed to prevent exposure of areas outside the elemental hologram. However, if used with such a band-limited diffuser, the object beam masking plate 60 may have an aperture larger than the size of the elemental hologram 110. An object beam 20 and a band-limited diffuser 45 that allow even illumination of an elemental hologram 110 by an object beam 20 need to be matched by a reference beam masking plate 65 with an aperture the size of the elemental hologram 110. Because the required matching of a object beam 20, a band-limited diffuser 45, and reference beam masking plate 65 to the size of a desired elemental hologram, it has been difficult to change the sizes of elemental holograms exposed by a hologram printer. Because of this lack of flexibility, prior art printers cannot easily print holograms having different sizes of elemental holograms, and are restricted to printing holograms with single, fixed-sized elemental holograms.
FIGS. 2-4 illustrate alternative prior art embodiments of optical systems that function in the same way as the optical system 89 depicted in FIG. 1.
In FIGS. 2-4, an object beam 20 is directed through a SLM 90 that has a sample image point 100 on its surface. The object beam 20 may be normal to the SLM surface or off-axis from the normal. SLM 90 may also have an array of pixels 95. LCD panels, cinematography film, and transparencies have been used as SLMs 90.
In FIG. 2, the object beam is directed through a simple diffuser 105, such as a section of ground glass, that scatters light. When a simple diffuser 105 is used, then an object beam masking plate 60 must be used to prevent exposing areas of the holographic recording material 70 outside of the elemental hologram 110 that are not meant to be exposed.
In FIG. 3, an object beam 20 is directed through a holographic optical element (HOE) 115. A HOE is a hologram that is specially designed to redirect light emanating from a source in a certain way. For instance, a HOE may be designed to act as a lens to converge light to a single point. As another example, a HOE may be designed to act as a band-limited diffuser that is paired with a lens to converge light over an area rather than at a single point. The HOE 115 depicted in FIG. 3 is one that is designed to evenly expose an area the size and shape of an elemental hologram 110. When such a HOE is used, an object beam masking plate 60 (shown in FIG. 2) need not be used at all or, if used, may have an aperture larger than the size of the elemental hologram 110 to be exposed.
In FIG. 4, an object beam 20 is directed through a band-limited diffuser, which may be a band-limited digital diffuser, 45 and a converging lens 55. The band-limited diffuser 45 depicted in FIG. 4 is designed to converge the object beam 20 over the area of elemental hologram 110. Thus, an object beam masking plate 60 (shown in FIG. 2) need not be used at all or, if used may have an aperture larger than the size of the elemental hologram 110 to be exposed.
In FIGS. 2-4, the sample image point 100 is an image point of the SLM 90 that is recorded in an elemental hologram 110 on a holographic recording material 70. Reference beam 25 is directed at the elemental hologram 110 such that the interference pattern formed by the interaction of the object beam 20 and the reference beam 25 may be recorded on the elemental hologram 110 on the holographic recording material 70.
To expose a two-dimensional array of elemental holograms, the prior art printer of FIG. 1 uses a mechanism for translating holographic film 69 that includes pulse controlled motors 71. Typically, the holographic recording material 70 in a prior art printer is photographic film. The film is held between the object beam masking plate 60 and the reference beam masking plate 65. Both masking plates 60 and 65 have apertures that are the size of the elemental holograms 110 being exposed. The masking plates 60 and 65 are moved by a solenoid 72. Pulse controlled motors 71 translate the film in two directions.
In the prior art system depicted in FIG. 1, the timing of the exposure sequence is controlled by a personal computer. Thus, the solenoid 72, as well as the pulse controlled motors 71 and the shutter 10, are controlled by the personal computer 85. In contrast, the images for the exposures are calculated off-line by a high-speed computer 87. The image calculations are transferred by an analog video signal to the LCD panel 50.
For a holographic stereogram to be reconstructed, an illumination source must be placed at an appropriate angle. If the illumination source is not placed correctly, a holographic stereogram will not be reconstructed or will appear with distortions, such as magnification distortions. Despite advances in holographic techniques and equipment, the display illumination geometry of a one-step, holographic stereogram remains a problem. The display illumination geometry, i.e., the placement of an illumination source with respect to a holographic stereogram, depends on the cumulative effect of the angles at which a reference beam exposed each of a holographic stereogram's elemental holograms. For example, if the angle at which all of the elemental holograms on a holographic stereogram are exposed to a collimated reference beam is constant, and if the surface of the holographic stereogram is flat, then the holographic stereogram needs a collimated illumination source to illuminate each of the elemental holograms from the appropriate angle if the hologram is to be properly reconstructed without defects such as magnification distortion.
Furthermore, in practice, it has been common to create reflection holographic stereograms which are meant to be illuminated with a diverging light source such as a point source. However, the prior art has not overcome the difficulty in designing a printer in which the angle of a reference beam is automatically and flexibly changeable to allow reconstruction by a point source and with minimal distortion.
In addition, it remains difficult to control the resolution or elemental hologram density of a holographic image. The sharpness of a holographic image depends on the image resolution and the extent of any blurring. Blurring can be caused by having a large illumination source, such as that of a long florescent light, illuminate a hologram. In addition, blurring can be caused by the large spectral spread of an illumination source. If an illumination source that is small and monochromatic, such as a laser source expanded through a microscope objective lens (i.e. a small, inexpensive, achromatic, high-power lens), is used, blurring may be minimized, and the sharpness of the holographic image will mainly depend on the image resolution of the hologram.
The image resolution of a three-dimensional image is defined as the volumetric density of individually distinguishable image points in an image volume. For one-step, holographic stereograms, including full-parallax and horizontal-parallax holograms, this resolution is usually not constant throughout an image volume. For small images of little depth, the variation of image resolution with depth is hardly noticeable. However, for holographic stereograms with significant depth, the variation in image resolution with depth can be very apparent.
As shown in FIG. 5, if the light from an illumination source 130 that is the same type of light source as that which generated the reference beam 25 that exposed an elemental hologram 110 illuminates the elemental hologram 110 from the appropriate conjugate angle. The reconstruction 125 of the sample image point 100, (shown in FIGS. 2-4), on the reconstruction 120 of the image of the SLM is formed at the same apparent distance and position relative to the elemental hologram 110 as it appeared to the elemental hologram 110 at the time of recording.
FIG. 6 shows lines drawn from the boundaries between neighboring elemental holograms 110 on a holographic, recording material 70 through the boundaries between neighboring reconstructed pixels 135 of the reconstructed image 120 of a SLM. The areas bounded between the lines drawn from the boundaries of at least two elemental holograms represent independently addressable volume elements, or voxels 140. A voxel 140 is a component unit which represents an arbitrary three-dimensional object or scene. Assuming that the elemental holograms 110 are larger than the SLM pixels, as shown in FIG. 6, the sizes of the voxels 140 increase with increasing distance from the surface of a reconstructed image of a SLM 120. If the sizes of the voxels 140 are too coarse relative to the desired detail size of a three-dimensional object or scene, the reproduced image will be poor or indiscernible. If the three-dimensional image of an object or scene extends over a wide range of depth, a variation in the sizes of the voxels will also be very undesirable because such a variation would lead to poor image quality.
Thus, hologram printers of the prior art have limitations that make them impractical for commercial purposes. In particular, these prior art printers suffer from: lack of ability to print full-color holographic stereograms; lack of ability to simultaneously expose multiple elemental holograms; lack of flexibility to quickly and easily adjust a hologram printer to print at different elemental hologram sizes; lack of flexibility to easily change the angle of a reference beam to a holographic recording material; lack of ability to control the resolution of a hologram; and lack of ability to create computer generated images which display animation or different images with a change of viewing position.